I. Field
The present invention relates generally to data communication, and more specifically to techniques for tracking residual frequency error and phase noise in an orthogonal frequency division multiplexing (OFDM) communication system.
II. Background
Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, and so on. These systems may utilize OFDM, which is a modulation technique capable of providing high performance for some wireless environments. OFDM effectively partitions the overall system bandwidth into a number of (NS) orthogonal subbands, which are also commonly referred to as tones, sub-carriers, bins, and frequency subchannels. With OFDM, each subband is associated with a respective sub-carrier that may be modulated with data.
In some OFDM systems, only ND subbands are used for data transmission, NP subbands are used for pilot transmission, and NG subbands are not used and serve as guard subbands to allow the systems to meet spectral mask requirements, where NS=ND+NP+NG. For example, an IEEE 802.11a system has 64 total subbands, 48 data subbands, 4 pilot subbands, and 12 guard subbands (i.e., NS=64, ND=48, NP=4, and NG=12). In each OFDM symbol period, one data modulation symbol (or simply, “data symbol”) may be transmitted on each of the ND data subbands, one pilot modulation symbol (or simply, “pilot symbol”) may be transmitted on each of the NP pilot subbands, and a signal value of zero is provided for each of the NG guard subbands. Each modulation symbol is a complex value for a specific point in a signal constellation for the modulation scheme used for that modulation symbol. The pilot symbols are known a priori at both the transmitter and receiver.
In an OFDM system, a transmitter initially codes, interleaves, and modulates a stream of information bits to obtain a stream of data modulation symbols. In each OFDM symbol period, ND data symbols, NP pilot symbols, and NG zero signal values (i.e., NS symbols for all NS subbands) are transformed to the time domain using an inverse fast Fourier transform (IFFT) to obtain a “transformed” symbol that contains NS complex-value chips. To combat frequency selective fading (i.e., a frequency response that varies across the NS subbands), which is caused by multipath in the wireless link, a portion of each transformed symbol is typically repeated. The repeated portion is often referred to as a cyclic prefix and includes Ncp chips. An OFDM symbol is formed by the transformed symbol and its cyclic prefix. Each OFDM symbol contains NS+Ncp chips and has a duration of NS+Ncp chip periods, which is one OFDM symbol period. The OFDM symbols are further processed and transmitted to a receiver.
The receiver performs the complementary processing, obtains NS+Ncp input samples for each received OFDM symbol, and removes the cyclic prefix from each received OFDM symbol to obtain a received transformed symbol. Each received transformed symbol is then transformed to the frequency domain using a fast Fourier transform (FFT) to obtain NS “received” symbols for the NS subbands. The received pilot symbols on the pilot subbands are typically used for various purposes such as channel estimation, timing acquisition, and phase/frequency tracking. The phase/frequency tracking may be implemented in various manners.
In one conventional phase/frequency tracking design, which operates on the input samples prior to the FFT, the receiver estimates frequency error in the input samples. The receiver then rotates the input samples to obtain frequency-corrected samples having the estimated frequency error removed. This open-loop design can estimate and correct for large frequency error, which can mitigate the deleterious effects of inter-carrier interference (i.e., interference from adjacent subbands). However, this design is not able to correct for residual frequency error/offset and phase noise. The residual frequency error can cause performance degradation, especially for larger-size packets and higher order modulation schemes with many points in their signal constellations.
In a second conventional phase/frequency tracking design, which operates on the received symbols after the FFT, the receiver estimates phase errors in the received pilot symbols. The receiver then averages the phase error estimates for all received pilot symbols for each OFDM symbol period to obtain a common phase correction value for all subbands. The receiver then corrects all received symbols for that OFDM symbol period with the common phase correction value to obtain phase-corrected symbols. This design can correct for residual frequency error. However, the use of a common phase correction value for all received symbols in each OFDM symbol period results in correlation among the phase-corrected symbols. This correlation typically does not affect the performance of an uncoded communication system. However, for a coded communication system that employs forward error correction coding (e.g., convolutional or turbo coding), the performance of the decoder at the receiver may be adversely affected by the correlation among the phase-corrected symbols provided to the decoder. This degradation is due to the fact that many decoders (e.g., Viterbi and turbo decoders) expect their input symbols to be uncorrelated for optimal performance. The degradation is especially noticeable for an additive white Gaussian noise (AWGN) channel, i.e., a wireless link with flat fading.
There is therefore a need in the art for techniques to track residual frequency error and phase noise without introducing correlation among the phase-corrected symbols.